Figure 27 shows an arrangement where the material is dried in different stages at various temperatures that can readily be controlled. The material can be handled in batches or continuously, and may be placed on trucks or slowly moving ribbons. This construction is mostly used for delicate substances that require certain temperatures for various stages of the drying process. It is in fact also a combination, of a number of single dryers, combined to one unit in order to save time, radiation losses, and labor.
All these dryers will require from 1J^ to 2 Ib. of steam per pound of moisture evaporated, and are therefore not so economical as dryers heated by hot gases or indirect dryers heated by steam and hot water.
FIG. 29.—Direct-heat dryer, fire or waste gases.
So far, we have been discussing the treatment of solids, but there are also cases where it is desirable and necessary to convert a liquid as concentrated solution into solids in one operation. For this purpose it is essential that the liquid is divided into very small particles (atomized) before it is brought into contact with the hot air or gases. Such a fine spray is produced by discharging the liquid from spray nozzles under very high pressure, or by running the liquid on a horizontal disc revolving
FIG. 29A.—Direct-heat dryer, steam or hot air.
at very high speed. Such a dryer is shown diagrammatically in Fig. 28. The air may also be cooled and reheated in the same manner as shown in Fig. 26. This type of apparatus is widely used fcr the drying of milk, but the steam consumption is high as compared with the vacuum-drum dryer, and losses of milk powder are unavoidable. A direct-heat dryer using either fire gases or waste gases is shown in Fig. 29; the revolving drum may be plain as shown or have a second concentric tube for the return of the gases. Temperatures are generally high, and the material must always travel with the current of the gases and not counter-current to prevent scorching and igniting. The efficiency is usually much better than in the air dryers, and 1 Ib. of coal will evaporate from 7 to 9 Ib. of water. In some cases, cold air is mixed with the gases